Embodiments of the present specification relate generally to a trolley interfacing device, and more particularly to a pre-charging unit employed in the trolley interfacing device.
Generally, a traction load, such as a railway car receives electrical power from trolley lines. The traction load includes pantographs that are used for coupling the traction load to the trolley lines for receiving the electrical power. Typically, a diesel generator is used as a voltage source to provide the electrical power to the traction load. However, in some scenarios, the traction load is driven by connecting the pantographs of the traction load to the trolley lines that are coupled to a direct current (DC) grid. The trolley lines are designed to convey/transfer a voltage that is higher than a design voltage, thereby entailing use of a buck converter to interface the trolley lines to the traction load. By way of example, the pantographs of the traction load that is electrically coupled to the trolley lines are used to provide the higher voltage from the trolley lines to the buck converter. Further, the buck converter converts this high voltage to a lower voltage before providing the voltage to the traction load. However, when the pantographs are coupled to the trolley lines, a large surge current flows from the DC grid to the buck converter via the trolley lines and the pantographs. This surge current results in voltage fluctuations in the DC grid and may damage the DC grid. Also, the surge current that flows to the buck converter may impose physical stress on the components in the buck converter and may damage the components in the buck converter.
In conventional systems, this surge current is prevented by precharging a capacitor in the buck converter through a resistor. Particularly, the resistor limits the current flowing from the DC grid to the capacitor, which in turn aids in slowing the charging of the capacitor, consequently preventing/minimizing the surge current at the capacitor. However, once the capacitor is charged, it is desirable to disconnect the resistor from the pantographs to avoid high electrical losses. In the conventional systems, to facilitate disconnecting the resistor from the pantographs, mechanical switches (MV switches) are positioned between the pantographs and the buck converter and across the resistor to aid in bypassing the resistor. However, these mechanical switches are heavy in weight, slow in switching, and also very expensive.